The tribesmen say Tuesday's attack started with a mortar round fired at the checkpoint, followed by heavy gunfire that killed most of its guards. The checkpoint is near the southern city of Ataq, the provincial capital of Shabwa.

They tribesmen spoke on condition of anonymity because they weren't authorized to speak to the media.

The troops guarding the checkpoint are part of a unit called the Elite Shabwa Force that was trained by the United Arab Emirates and deployed last year to the region. It declared victory over al-Qaida's branch in Yemen, which used Shabwa as a safe haven. The UAE is part of a Saudi-led coalition battling Yemen's Shiite rebels who control the country's north.

Meanwhile, UAE-trained forces in the city of Aden — some 400 kilometers, or 250 miles, from Shabwa— have battled Yemeni government forces there and seized control of a district where the presidential palace is located.

—Ahmed Al-Haj in Sanaa, Yemen;

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10 a.m.

Yemeni security officials say the prime minister is preparing to flee the country for Saudi Arabia after separatists seized the area around the presidential palace in the southern port city of Aden in fierce battles overnight.

The officials say fighters loyal to the so-called Southern Transitional Council fought all way to the gates of the Palace of Maashiq in the district of Crater in Aden, forcing President Abed Rabbo Mansour Hadi's troops to abandon their positions.

They said Hadi's prime minister and several Cabinet members would leave imminently to Riyadh.

The palace is the seat of Yemen's internationally backed government. The separatist forces did not enter the palace itself and were stopped by Saudi Arabian troops who have been guarding the palace for the past months.